It is known to employ a thermostatic capsule to move a shuttle to open or close small gaps on either side of the shuttle. It is convenient for this shuttle to be circular with the gaps being annular, but this gives rise to other problems.
Conventionally the thermostatic capsule has:                a circularly cylindrical, copper sheath containing thermostatic wax,        a location flange at the inner end of the copper sheath,        a spigot extending from the location flange oppositely from the sheath and        a push rod extending from the spigot by an amount varying in accordance with the ambient temperature at the copper sheath.        
The inlet to the annular gaps is conveniently radially inwards. Normally, the cold water flows in upstream of the hot water, whereby as the combined flow is turned to flow axially along the copper sheath of the thermostatic capsule, the cold water tends to flow closer to the capsule than the hot water. At least this is the situation close to the flange of the capsule that is its most sensitive part. Further downstream, the flow mixes better, so that the tail end of the capsule experiences a more representative temperature of the actual mixed water flow. Should the normal situation be reversed and hot water flow in upstream of the cold water, the same problem of the capsule not experiencing mixed water temperature at its flange end of the copper sheath will obtain.